U.S. patent number 4,559,814 [Application Number 06/585,194] was granted by the patent office on 1985-12-24 for thermal air flow meter.
This patent grant is currently assigned to Hitachi, Ltd.. Invention is credited to Kanemasa Sato, Sadayasu Ueno.
United States Patent |
4,559,814 |
Sato , et al. |
December 24, 1985 |
Thermal air flow meter
Abstract
A thermal air flow meter is equipped with a flow rate detector
utilizing the phenomenon that heat is carried away in proportion to
flow rate. The flow rate detector comprises a support, a
heat-sensitive resistor formed on the support and leads attached to
both ends of the support. To make the temperature distribution
uniform, the heat-sensitive resistor is formed in such a manner
that the resistance per unit length of the heat-sensitive resistor
at either end of the support is greater than the resistance per
unit length of the heat-sensitive resistor at the center of the
support. This arrangement can provide a thermal air flow meter
which has a rapid response to changes in flow rate.
Inventors: |
Sato; Kanemasa (Katsuta,
JP), Ueno; Sadayasu (Katsuta, JP) |
Assignee: |
Hitachi, Ltd. (Tokyo,
JP)
|
Family
ID: |
12456449 |
Appl.
No.: |
06/585,194 |
Filed: |
March 1, 1984 |
Foreign Application Priority Data
|
|
|
|
|
Mar 7, 1983 [JP] |
|
|
58-35957 |
|
Current U.S.
Class: |
73/114.34;
73/204.26; 73/204.27 |
Current CPC
Class: |
G01F
1/698 (20130101); G01F 1/6842 (20130101) |
Current International
Class: |
G01F
1/696 (20060101); G01F 1/684 (20060101); G01F
1/698 (20060101); G01F 001/68 () |
Field of
Search: |
;73/204,118
;338/25,296 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Goldstein; Herbert
Attorney, Agent or Firm: Antonelli, Terry & Wands
Claims
What is claimed is:
1. In an internal combustion engine a thermal air flow meter
including a flow rate detector for detecting a proportion of a flow
rate of air in dependence upon heat carried away by the air the
flow rate detector comprising a support means, means disposed on
said support means for increasing a response time of the thermal
air flow meter and improving an accuracy of the measurement of flow
rate including a heat-sensitive resistor formed on said support
means in such a manner that the resistance per unit length of said
heat-sensitive resistor at either end of said support means is
greater than the resistance per unit length of said heat-sensitive
resistor at a center of said support means, and leads attached to
both ends of said support means and electrically connected to said
heat sensitive resistor.
2. The thermal air flow meter as defined in claim 1, wherein said
heat-sensitive resistor is linear and is wound around the outer
periphery of said support means in such a manner that said resistor
is wound densely at either end of said support means and coarsely
at the center area thereof.
3. The thermal air flow meter as defined in claim 2, wherein said
support means is cylindrical, said leads are inserted into and
attached to said cylindrical support, and said heat-sensitive
resistor is wound densely at least over the portions corresponding
to the portions at which said lead are inserted into said
cylindrical support.
4. The thermal air flow meter as defined in claim 1, wherein said
heat-sensitive resistor has a film-like form and is constructed by
trimming a film formed on the surface of said support means, and
the pitch of said trimming is such that it is dense at either end
of said support means and coarse at the center of said support
means.
Description
BACKGROUND OF THE INVENTION
This invention relates to a thermal air flow meter, and, more
particularly, to a thermal air flow meter suitable for measuring
the quantity of air taken in by an internal-combustion engine.
Various systems for measuring the quantity of air taken in by an
internal-combustion engine have been known in the past, such as
movable vane types, types utilizing Karman's vortex sheets, and so
forth. Thermal air flow meters disclosed in, for example, U.S. Pat.
Nos. 3,747,577 and 4,304,128 have recently gained wide application
because they usually have a rapid response and can measure the mass
flow rate of the air. A thermal air flow meter of the
aforementioned type includes a platinum wire of a diameter of
between 70 .mu.m and 100 .mu.m stretched within an intake pipe of
an internal combustion engine to act as a flow rate detector. A
disadvantage of this construction resides in the fact that the
thermal air flow meter does not have sufficient durability when the
internal-combustion engine is running badly, and the flow meter
undergoes mechanical damage due to backfiring.
In, for example, U.S. Pat. No. 4,264,961, an improved thermal air
flow meter solving this problem is proposed wherein a part of the
air flowing through the intake pipe is led into a by-pass pipe, and
the platinum wire, acting as the flow rate detector, is mounted in
this by-pass pipe. Since the by-pass pipe has a maximum diameter of
1 cm, the flow rate detector must also be compact. However, since
the flow meter measures flow rate by utilizing the phenomenon that
the resistance of platinum wire varies with temperature, a higher
sensitivity can be obtained by a higher-resistance platinum wire.
Accordingly, the flow rate detector is constructed by winding
platinum wire around the outer periphery of a piece of insulating
material to make the flow meter compact and increase its
resistance. With this construction, however, another problem occurs
in that the response is lower than that of the system described
above, because of the heat capacity of the bobbin used as the
support. This problem is not limited to the type of meter which
utilizes a by-pass pipe, but also to any meter which utilizes a
compact flow rate detector.
The response problem is more critical when the thermal air flow
meter described above is, for example, used for a single-point fuel
injection system.
In single-point fuel injection, a single injection valve is
provided at a point at which the intake pipes of the engine join,
and hence the distances from the fuel injection position to the
cylinder inlets is longer than those in multi-point fuel injection,
and the time taken for the fuel to arrive at the cylinders is
longer. Similarly, since the distance from the fuel injection valve
to each cylinder differs from cylinder to cylinder, delicate
matching must be carried out. Although attempts have been made to
compensate for the difference by use of computer software, these
have not been entirely successful. After all, in single-point
injection, only one injection valve distributes fuel to each
cylinder so that delicate matching must be made whenever the model
of the car, and hence the shape of the intake pipes, changes.
Particularly during acceleration or during high-speed operation,
the detection accuracy must be improved so that the pulsating flow
of intake air in the engine can be followed precisely, using a
highly accurate flow rate sensor.
This problem of response occurs not only in the control of an
internal-combustion engine, such as in the single-point fuel
injection system, but also in the measurement of flow rates in
general if changes in such flow rates are rapid.
It is therefore an object of the present invention to provide a
thermal air flow meter which is equipped with a compact flow rate
detector, but which still provides a rapid response.
In a thermal air flow meter equipped with a flow rate detector
utilizing the phenomenon that heat is carried away in proportion to
the flow rate, the thermal air flow meter in accordance with the
present invention is characterized in that the flow rate detector
comprises a support, a heat-sensitive resistor formed on the
support, and leads attached to both ends of the support. The
heat-sensitive resistor is formed on the support in such a manner
that the resistance per unit length of the heat-sensitive resistor
at either end of the support is greater than the resistance per
unit length of the heat-sensitive resistor at its center.
Various experiments and studies have been carried out using
conventional flow rate detectors of the wound type, and have
clarified the following points.
In a conventional flow rate detector, platinum leads which function
both as supports and conductors are attached to both ends of a 2
mm-long bobbin made of an insulating material, and platinum wire is
wound at constant pitch onto this bobbin. When a current flows
through the platinum wire and the heat thus generated is controlled
so that the wire is at a predetermined temperature, the temperature
distribution is such that it is highest at the center and drops
towards the leads. Accordingly, when the set temperature for the
flow rate detector is, for example, 170.degree. C., (this can be
effected by making the resistance of the flow rate detector a
predetermined value), the maximum temperature at the center is
about 250.degree. C.
The reason why the temperature distribution is so large can be
attributed to the following. In the initial stages when the current
starts to flow through the resistance wire, the quantity of heat
generated per unit length is the same; but as the heat is
transferred, temperature differences occur between the glass
coating, the bobbin, and the leads that are in contact with the
winding. These temperature differences change the resistance of
each part of the resistance wire. For instance, the resistance
rises locally at the center, further increasing the quantity of
heat generated. When specific structures are examined, it is first
of all obvious that the bobbin center is hollow while the two end
portions hold the leads and adhesive for the leads, so that they
have different volumes which induce differences in heat capacity.
Secondly, heat sinks are generated from both end portions because
leads of a precious metal are fitted.
In accordance with the present invention, since the resistance of
the bobbin is smaller at the center thereof than that at either
end, heat generation is less at the center of the bobbin and is
more at either end, so that the present invention can provide a
thermal air flow meter in which the temperature distribution along
the flow rate detector can be made substantially uniform, and which
has a rapid response.
The resistance of the heat-sensitive resistor per unit length at
the center of the bobbin can be easily made different from that at
either end by winding the resistor onto a support coarsely at the
center and densely at either end, if the heat-sensitive resistor is
a wire, or by making the trimming pitch dense at either end and
coarse at the center if it is produced by trimming after the
formation of a film.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a section through a thermal air flow meter measuring the
quantity of air taken in by an internal-combustion engine, in
accordance with one embodiment of the present invention;
FIG. 2 is a plan view of a portion of the thermal air flow meter of
FIG. 1;
FIG. 3 is a bottom plan view of FIG. 1 with the skirt portion is
removed;
FIG. 4 is a circuit diagram of the thermal air flow meter
constructed in accordance with one embodiment of the present
invention;
FIG. 5A is a photomicrograph side view of the flow rate detector of
the thermal air flow meter in accordance with one embodiment of the
present invention at a magnification of fifty;
FIG. 5B is a longitudinal cross-sectional view of the detector of
FIG. 5A at a magnification of fifty;
FIG. 5C is a graph of the temperature distribution along a flow
rate detector;
FIG. 6 is a graph of the response characteristics of one embodiment
of the present invention;
FIG. 7 is a graph of the response characteristics of a prior art
flow meter;
FIG. 8 is a graph comparing the characteristics of a prior art flow
meter and the flow meter of the present invention;
FIG. 9 is a side view of the flow rate detector of a thermal air
flow meter in accordance with another embodiment of the present
invention; and
FIG. 10 is a longitudinal cross-sectional view of the detector of
FIG. 9.
DETAILED DESCRIPTION
Referring now to the drawings wherein like reference numerals are
used throughout the various views to designate like parts and, more
particularly, to FIGS. 1-3, according to these figures, a thermal
air flow meter suitable for measuring the quantity of air taken in
by an internal-combustion engine includes a body portion 10, formed
by aluminum die casting, a head portion 11 fitted onto the body
portion 10, with an air cleaner (not shown) being connected
upstream of the head portion. A skirt portion 12, formed by
aluminum die casting is connected at downstream end thereof
connected to an engine (not shown). Part of each of the inner walls
of the portion 10 and skirt portion 12 forms form a venturi portion
14. A by-pass passage 18 is formed in the body portion 10 as well
as a main passage 16, with the by-pass passage 18 including a
linear portion 18a which is parallel to the main passage 16, and a
curved portion 18b surrounding the main passage 16.
As shown most clearly in FIG. 3, the curved portion 18b of the
by-pass passage 18 surrounds about 3/4 of the main passage 16. As
shown in FIG. 1, the outlet 18c of the by-pass passage 18 is shaped
as a laterally elongated slot. A heat-sensitive resistor 20, for
measuring the air flow rate, and a heat-sensitive resistor 22, for
compensating for the air temperature, are positioned as sensors
inside the linear portion 18a of the bypass passage 18. The two
heat sensitive resistors 20, 22 are attached to support pins which
are connected to an electric circuit within a circuit case 26,
through a piece of heat-insulating material 24. Hot wire resistors
or hot film resistors may be used as the heat-sensitive resistors
20, 22, these resistors will be described elsewhere in further
detail.
In the electric circuit of FIG. 4, a voltage V.sub.B, from a power
source such as a car, is applied to the heat-sensitive resistor 20
for measuring the flow rate, through a transistor 30. A resistor 32
is connected in series with the heat-sensitive resistor 20, and is
used for measuring a current flowing through the heat-sensitve
resistor 20. The voltage across the ends of the heat-sensitive
resistor 20 is divided by resistors 34, 36. The divided voltage is
applied to one of the input terminals of a differential amplifier
40 through a resistor 38. The negative input of an amplifier 42 is
grounded by a resistor 44. The heat-sensitive resistor 22 for
temperature compensation and a resistor 46 function as feedback
resistors for the amplifier 42. The output of the amplifier 42 is
applied to the positive input of the amplifier 40 through a
resistor 48. Accordingly, the amplifier 40 produces an output by
amplifying the divided voltage across the two ends of the
heat-sensitive resistor 20 and the voltage at a junction B between
the heat-sensitive resistor 20 and the resistor 32 by an
amplification factor in response to the ambient temperature, so
that it is equal to the voltage amplified by the amplifier 42. The
output of the amplifier 40 is applied to the base of the transistor
30 and the amplifier 40 produces an output such that the output is
equal to this input, thereby controlling the transistor 30. As a
result, the heat-sensitive resistor 20 is maintained at a
predetermined temperature of about 170.degree. C. higher than the
ambient temperature of the heat-sensitive resistors 20, 22.
A series circuit of a resistor 52 and a variable resistor 54 is
connected to a constant voltage source 50, with a junction of the
resistors 52, 54 connected to the negative input of the amplifier
42 through a resistor 56, and with the resistors 52, 54 being used
for offset adjustment. The output of the constant voltage source
50, connected to the battery voltage V.sub.B, is divided by
resistors 58, 60, with a junction of the resistors being connected
to the positive input of the amplifier 40 by a diode 62. This
circuit is a starting circuit, when the key switch of a vehicle is
turned on, a predetermined voltage is applied through the diode 56
so that the two inputs to the amplifier 40 become different, and an
output is forced from the amplifier 40. The junction of the
heat-sensitive resistor 20 and the resistor 32 is connected to an
amplifier 64. The amplifier 64 produces an output which is the
voltage across the two ends of the resistor 32. Since the
resistance of the resistor 32 is constant, the output V.sub.S1 of
the amplifier 68 indicates the current flowing through the resistor
32, that is, the current flowing through the heat-sensitive
resistor 20, and is a signal indicating the air flow rate. The
output V.sub.S2 of the amplifier 40 also indicates the current
flowing through the heat-sensitive resistor 20, and is an air flow
rate signal.
As shown in FIGS. 5A, 5B, in a heat-sensitive resistor acting as a
flow rate detector, platinum leads 102, 104 are inserted into
either end of a hollow alumina bobbin 100 and are bonded by
borosilicate glass 106, 108, respectively. A platinum wire 110 is
wound around the outer periphery of the bobbin 100 in such a manner
that the coil produced is coarser at the center and denser at
either end. In other words, this winding method is different from
the conventional uniform winding. Both ends 112, 114 of the
platinum wire 110 are spot-welded to the leads 102, 104,
respectively. Lead glass is also applied around the platinum wire
110 and is baked at 600.degree. C. to form a 10.mu. to 20.mu.-thick
protective film 116. Both ends of the leads 102, 104 are
spot-welded to support pins 118 (the right-hand support pin is
omitted in the drawing).
The bobbin 110 may be made of any electrically insulating material,
such as, for example, magnesia and zirconia, besides alumina. Other
metallic wires can be used as the leads 102, 104, so long as they
are electrically conductive. Platinum paste or metal alloys can be
used as the adhesive 106, 108. The platinum wire 110 may be
substituted by other metal wires so long as they are electrically
conductive and have a large coefficient of thermal resistance, but
from the aspect of stability, platinum is preferred.
The dimensions of each part of the embodiment shown in FIGS. 5A and
5B are as follows:
Total length (L) of bobbin 100: 2 mm
Length l.sub.1 at each end of bobbin 100 at which dense winding of
platinum wire is difficult: 0.1 mm
Length l.sub.2 at each end of bobbin 100 over which platinum wire
is wound densely: 0.45 mm
Length l.sub.3 of the center of bobbin 100 over which platinum wire
is wound coarsely: 0.9 mm
Length l.sub.4 of leads 102, 104 inserted into ends of bobbin 100:
0.5 mm
Outer diameter d.sub.1 of bobbin 100: 0.5 mm
Inner diameter d.sub.2 of bobbin 100: 0.3 mm
Outer diameter d.sub.3 of leads 102, 104: 0.15 mm
Outer diameter d.sub.4 of support pins 118: 0.8 mm
The method of winding the platimum wire 110 will now be described.
The platinum wire 110 is wound fourteen times around the
densely-wound end portions of the bobbin 100. The platinum wire 110
is 20.mu.-thick and the spacing between one loop of platinum wire
and the next is 12 .mu.m. On the other hand, the platinum wire 110
is wound seven times around the coarsely-wound portions at either
end of the bobbin so that the spacing between one loop of platinum
wire and the next is 108 .mu.m. Incidentally, it is preferable to
wind the platinum wire completely up to both ends of the bobbin
100, but a sufficiently tight coil can not always be obtained at
the start of winding, because winding is difficult there.
The length of the center of the bobbin 100 is about 50% of the
total length, and is substantially 50% when the two end portions
are added. Since the bobbin is hollow at the center, however, the
volumetric ratio of the center to that of the two end portions is
40:60 (%). Accordingly, the ratio of their heat capacities is also
substantially this ratio. Furthermore, since the leads 102, 104 are
connected to the two end portions, heat sinks must be considered.
For these reasons 20% of the total length of the platinum wire is
wound around the center, that is, that portion accounting for 20%
of the total resistance, 40% of the total length of the platinum
wire, or 40% of the total resistance, is wound around each end
portion.
As a result, the temperature distribution of the flow rate detector
is such as shown in FIG. 5C, the temperature difference between the
center and either end portion is at most 10.degree. C., and the
maximum temperature has dropped to about 200.degree. C. The
response of this embodiment was measured, with the results shown in
FIG. 6. Similarly, the response of a prior-art detector produced by
winding platinum wire at equal pitch (thirty-five coils with a
spacing of 35 .mu.m due to the uniform winding) is shown in FIG.
7.
The graphs of FIGS. 6 and 7 show the response outputs of the flow
meters when the air flow rate was increased in a single step from 0
Kg/H to 200 Kg/H at time 0, and the response output of the flow
meters when the air flow rate was similarly decreased in a single
step from 200 Kg/H to 0 Kg/H. Numerals in the graphs show the
response time 3.tau. after the change in flow rate until 95% of the
full scale was reached. In the prior art flow meter, the rise
response time was 1700 ms and the fall response time 107 ms. In the
present embodiment, the rise and fall response times were improved
to 105 ms and 60 ms, respectively. The rise response time, in
particular, was reduced by about 1/7.
Since the rise response time is reduced, the difference between the
rise response time and the fall response time is small, so that
when a pulsating flow in the engine is being detected, the flow
rate can be detected with an accuracy approximately that of the
mean value, and the detection accuracy of the flow rate can be
improved.
Since the heat transferred to the air flow is increased, the
sensitivity of a heat-flow sensor can be improved. FIG. 8 shows two
characteristics, that is, the characteristics of a prior art sensor
and those of a sensor with coarse winding, in which the square of
the current I.sub.H applied to the resistor is plotted along the
ordinate and the square root of the flow rate Q along the abscissa.
The power consumption is reduced by 15% on the low flow-rate side
for the sensor with the coarse central winding, in comparison with
that of the prior art. The heat transferred to the air flow is
improved by about 20% in terms of gradient, and the sensitivity can
be improved.
Experiments were carried out involving changes in the winding
ratios. The results are shown in Table 1.
In the prior art flow meter, the wire is wound thirty-five times at
equal pitch. Example 1 is the embodiment of the invention described
above in which the wire is wound densely fourteen times at either
end and coarsely seven times at the center. It was found to be
difficult to wind the wire more than fourteen times within a zone
of the length of 0.45 mm from either end (because neighboring
platinum wires come into contact with each other). Accordingly, the
experiment was repeated by winding the wire thirteen times on
either side and nine times at the center, without changing the
overall number of windings in Example 2. This experiment provided a
rise response time of 180 ms and a fall response time of 83 ms,
these values are superior to those of the prior art but are
inferior to those of Example 1. In other words, with Example 2, a
heat sink occurs from either end, and the temperature is higher at
the center than at either end so that variations occur in the
temperature distribution. In Example 3, the number of windings was
the same as those of Example 1, but there were two more coils at
the center to increase slightly the temperature at the center. With
this example, the response characteristics were substantially the
same as those of Example 1.
In other words, the response can be improved by winding the wire
more densely at either end than at the center. The density of the
winding is preferably determined experimentally in accordance with
the overall heat capacity distribution and the magnitude of the
heat sink at each end.
TABLE 1 ______________________________________ Each end Center
Total Rise Fall ______________________________________ Prior art
11.3 11.3 35 1700 ms 107 ms Example 1 14 7 35 105 60 Example 2 13 9
35 180 83 Example 3 14 9 37 100 56
______________________________________
The examples described above had hollow bobbins in order to
restrict any increase in the total heat capacity, and facilitate
the fitting of the leads. However, the bobbin can be a cylindrical
bobbin, or a support on a flat sheet. In such a case, the heat
capacity is substantially uniform so that heat sinks must be
primarily considered.
In the embodiment of FIGS. 9 and 10, leads 202 and 204 are inserted
into both ends of a hollow bobbin 200 of an insulating material,
and the bobbin 200 and the leads 202, 204 are bonded by adhesive
206, 208. A platinum film 210 is provided around the outer
periphery of the bobbin 200. The platinum film 210 is spirally
trimmed by a laser so that the trimming pitch is dense at either
end and is coarse at the center thereof. In this manner, the
resistance at either end can be made large and the resistance at
the center small, even if the film is trimmed by the laser to a
constant width. This arrangement makes the temperature distribution
in the axial direction of the bobbin 200 constant, in the same way
as in the embodiment described above. In this embodiment, the
connection of the platinum film 210 to the leads 202, 204 can be
done easily by forming the platinum film 210 so that it reaches the
leads 202, 204.
In a thermal air flow meter of the type in which a platinum film is
formed on a flat sheet, the platinum film 210 is trimmed into a
zigzag such that its pitch is dense at either end and coarse at the
center. Thus, substantially the same effects as those of the
embodiments described above can be obtained.
In accordance with the present invention, the temperature
distribution of the flow rate detector can be made substantially
uniform to improve the heat transfer quantity thereof with respect
to the air flow.
Accordingly, the sensitivity as well as the accuracy of the
measurement of the flow rate can be improved.
The rise response time can be made faster by reducing the highest
temperature of the bobbin.
Since a value approximately the same as the true value of a
pulsating quantity can be detected by reducing the difference
between the rise and fall response time, the accuracy of measuring
flow rate can be improved.
* * * * *